A method and an arrangement for the above-mentioned purpose are described in U.S. Pat. No. 4,461,258. The arrangement includes at least one characteristic field for the values of a control parameter which can be addressed via values of pregiven operating variables such as engine speed and load. In this way, control frequency and/or control condition can be adapted to the particular actual operating state of the controlled engine so that an especially low discharge of toxic gas from the engine takes place, that is, a discharge from a catalytic converter following the engine.
U.S. Pat. No. 4,461,258 discloses that the average values for .lambda. can be shifted in a clearly defined manner such that the individual exhaust gas components are substantially adapted to the desired value at all operational points.
Values of control parameters which lead to a minimum discharge of toxic gas are not only dependent upon the actual operating state of the engine but also upon the way in which this operating state was reached. For example, if an operating state having a high engine output and therefore a high engine temperature is reached from a very low power output, another time-dependent pattern of exhaust gas discharge from the engine is to be expected in accordance with start value and time constant (because of the dependence of the exhaust gas composition on the combustion temperature) than if the same operating state is reached starting from such a power output which is very similar. The exhaust gas temperature is also dependent upon the combustion temperature, which, in turn, determines the temperature of the catalytic converter. This temperature influences the conversion characteristics of the catalytic converter. In the known arrangement, the values stored in the particular characteristic field are so applied that they collectively lead to minimum toxic gas discharge from the catalytic converter for typical predetermined operating sequences.
Since the above-mentioned method and arrangement have been known, the need has been present to provide a corresponding method and arrangement which make it possible to further drop the exhaust gas discharge.